I'm working on a classroom activity on whales, and I would like to tell the
pupils also about the whale's ancestors. I found lots of information but
they differ very much from each other... (land relatives of whales as a
group of mammals called mesonychians? Or whales as closely related to the
modern hippopotamus, having an ancestor in common?)? Is there a widely
accepted hypothesis?
When did the first modern whales live?
Can you tell me something about the phylogenetic relationship between tooth
and baleen whales?

Thank you very much for your help!
Annette (from Germany)

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Annette,

As of a Nov, 1999 article in ScienceNews, which summarized the most recent
aspects of the ongoing debate over whale
volution,( http://www.findarticles.com/m1200/19_156/57828404/p1/article.jhtm
l), there is no clear agreement yet. For decades, the "mostly" accepted
hypothesis has been that whales evolved into the ocean about 50 millions
years ago from land-dwelling Condylarths, which includes a group of animals
now known as "even-toed ungulates" (which means they have two hooves on each
foot, rather than one). This means that whales are related to modern
even-toed's like, for example hippos. In fact, recent genetic evidence
supports the idea that the hippo is the closest relative of the cetaceans.
But, as you will read in the article, fossil evidence supports a close
ancestry with the Mesonychids, a wolf-like animal with hooves, which is NOT
closely related to hippos. So, the debate continues....

Of the modern whales and dolphins, the sperm whale is probably the oldest,
appearing in the fossil record as long as 25 millions years ago. Most other
modern cetaceans are younger than that at 5 to 15 millions years old.

Toothed and baleen whales appear to have evolved from the same ancestors.
Toothed whales appeared first, then baleen whales split off from the main
group about 25 to 35 million years ago.